The present invention relates to a shock absorber capable of generating a damping force by controlling a flow of hydraulic fluid in a cylinder during a stroke of a piston rod, a tube provided with a branch tube capable of being used for the shock absorber, and a method for making them.
For example, Japanese Patent Application Public Disclosure No. H11-159563 discusses a tubular shock absorber mounted on a suspension apparatus of a vehicle such as an automobile. This shock absorber has a triple tube structure including a cylinder, an outer tube disposed around the cylinder, and a cylindrical member (a separator tube) disposed between the cylinder and the outer tube. An annular passage is defined between the cylinder and the cylindrical member. Further, the cylindrical member is configured as a tube provided with a branch tube by including an integrally formed branch tube, which cylindrically protrudes from a side wall of the cylindrical member radially outwardly while being in communication with the annular passage.
The shock absorber disclosed in the above-described Japanese Patent Application Public Disclosure No. H11-159563 uses the branch tube integrally formed at the side wall of the cylindrical member as a passage of hydraulic fluid. According to this configuration, the cylindrical member and the branch tube have to have a sufficient pressure resistance against the hydraulic fluid, which is supposed to be highly pressurized. Further, the cylindrical member also has to have a sufficient fatigue strength because the cylindrical member is supposed to repeatedly receive a load due to a pressure change according to a stroke change of the piston rod between an extension side and a compression side. Then, while satisfying these strength-related requirements, the wall thickness is desired to be thinned to reduce the weight of the shock absorber.